EP0748395B1 - Process for producing layers of cubic boron nitride - Google Patents

Abstract

PCT No. PCT/DE95/00315 Sec. 371 Date Sep. 4, 1996 Sec. 102(e) Date Sep. 4, 1996 PCT Filed Mar. 3, 1995 PCT Pub. No. WO95/23879 PCT Pub. Date Sep. 8, 1995Process for producing wear-resistant layers of cubic boron nitride or wear-resistant layers containing cubic boron nitride by sputtering with RF or DC voltage in the operating mode of an unbalanced magnetron, in which the plasma is generated by DC arc discharges or DC operated magnetron cathodes. The initial target for the production of the layer from which the material is removed comprises electrically conductive material containing boron, preferably boron carbide, and, in the process, the reactive process is conducted with the addition of N2 and Ar in such a way that the necessary stoichiometric ratio BiN in the layer can be adjusted.

Description

The invention relates to the field of the production of wear-resistant layers or the production of layers with certain wear properties. It relates to a process for the production of wear-resistant layers made of cubic boron nitride (cBN) or connecting layers with a high cBN content.

The thermodynamically unstable cBN is characterized by a high hardness that is only surpassed by diamond and has other properties that are similar to those of diamond. These include in particular the good thermal conductivity, the excellent tribological properties and last but not least the good transparency in the visible (n = 2.2), as well as an extremely large band gap of> 6.4 eV, which means that cBN also has high potential as a semiconductor material.

In the field of manufacturing technology, cBN has the essential advantage over diamond that it is inert to steel and thus enables the machining of workpieces containing iron. This important area of production is not accessible to diamond tools due to chemical wear of the diamond.

Since it was mainly Japanese and American scientists in the 80s who fundamentally demonstrated that the cubic phase of boron nitride can also be produced with the aid of plasma-activated processes, efforts have been made worldwide to produce cBN layers.

For example, in "Journal of Applied Physics" 72 (2), July 15, 1992, page 504 and in "Journal of Material Research" Vol 8, No. 6, Jun. 1993, described the production of cBN layers, using an ion beam assisted vapor deposition technique (IBAD).

Furthermore, in "Diamond and Related Materials" 2 (1993), page 512, a method is described which works with the aid of ion beam guns, an ion source being used to atomize the targets consisting of boron or hexagonal boron nitride (hBN) and a second ion cannon Bombardment of the film deposited on a substrate takes over.

In "Surface and Coating Technology" 43/44 (1990), pages 145-153, a reactive lonplating process is also described, in which boron is evaporated using an electron beam evaporator in an Ar / N ₂ atmosphere which is additionally ionized . The layer that is formed lies on a substrate holder with negative potential, so that the growing layer is bombarded with the Ar and N ₂ ions present.

In the method described in "Thin Solid Films" 224 (1993), pages 46-51, the cBN layers are produced with the aid of an ion-assisted laser pulse method. An hBN target is used. The ion bombardment is carried out by applying a DC voltage potential.

In "Japanese Joumal of Applied Physics" Vol.- 30, No. 2, Febr. 1991, pages 344-348 described method works with a microwave electron cyclotron resonance (EZR) deposition, wherein BN is formed from the gas phase Ar / N ₂ / B ₂ H ₆ and an HF to the substrate -Bias (high frequency bias) is placed.

Finally, in "Japanese Joumal of Applied Physics" Vol. 29, No. 7, July 1990, pages 1175-1177, a method for producing layers containing cBN is shown, in which an hBN target is atomized in an inert high-frequency plasma (HF plasma). The substrate is also held at negative potential with an HF bias, so that bombardment with Ar ions takes place.

For the sake of completeness, reference is also made to DE-OS 38 10 237, US Pat. No. 4,412,899, US Pat. No. 4,415,420, US Pat. No. 4,683,043 and US Pat. No. 5,096,740, which likewise contain different processes for the preparation of cBN or cBN-containing layers are shown.

The PVD processes described in the prior art uniformly use either boron or hexagonal boron nitride (hBN) as the target or vapor deposition material. All of these methods had considerable disadvantages in terms of economic application. With this method, only relatively small areas can be coated with a low deposition rate.

The known CVD-EZR processes have the considerable disadvantage that the deposition can only take place at relatively high substrate temperatures of> 600 ° C. moreover, due to the limited extent of the EZR plasma, only relatively small substrates can be coated.

Nowadays, PVD processes are essentially used in hard film production in general, the plasma being generated for economic reasons with the help of direct current arc discharges (ARC process) or with the help of magnetron cathodes operated with direct current. In addition to essential cost advantages in the apperative area, direct current operation also has the particular advantage that large-area coating systems can be implemented with it.

With regard to the production of cBN layers or layers containing cBN, these processes have the disadvantage that the use of electrically conductive starting targets is absolutely necessary and the production of cBN layers according to the prior art only from the electrically non-conductive B- or hBN starting materials or a gas phase containing B ₂ H ₆ is possible. This inexpensive state-of-the-art method for the production of cBN layers cannot therefore be used.

The object of the invention, which includes proposing a method for producing hard and wear-resistant cBN layers or layers containing cBN, which does not have all the disadvantages of the prior art, is therefore to specify a method of the type mentioned which, in addition to a high degree Coating rate also allows the deposition at low substrate temperatures, which is not limited to small substrates, and which is economically advantageous and inexpensive to operate.

In addition, it is an object of the invention to provide a method of the type mentioned, with which a large-area coating with high deposition rates is possible, that is, it is also an object of the invention to create the conditions for the PVD method in which the Plasma is generated with the aid of direct current arc discharges or with the aid of magnetron cathodes operated with direct current, for which cBN layers or layers containing cBN can be used.

As a result, it is therefore the object of the invention to provide a starting target material with an electrically sufficient conductivity, from which cBN layers or layers containing cBN can be produced, and the suitable process parameters for this.

Finally, it is still an object of the invention to specify a possible application of the method of the type to be proposed.

According to the invention, the tasks are solved on the process side, as described in claims 1 to 16 and on the application side, as described in claims 17 to 22.

Surprisingly, and it was not at all to be expected, it was found that, if the process conditions selected are observed, cBN layers or layers containing cBN can also be obtained using boron carbide (B ₄ C) targets which contain approximately 20% carbon. let it separate. Advantageously, before the actual layer application, a target cleaning phase and a simultaneous ion etching cleaning of the substrates, separated by a mechanical screen, take place.

In the method for producing wear-resistant layers from cBN or cBN-containing wear-resistant layers by means of PVD or similar methods, in which the plasma is generated with the aid of direct current arc discharges (ARC method) or with the aid of magnetron cathodes operated with direct current, an output target is inventively used , on which the material removal for the layer production takes place, is made of electrically conductive material. This target preferably consists of boron carbide, advantageously in the composition range from 90at% to 70at% boron and 10at% to 30at% carbon (eg B ₄ C). In another process variant, B- and / or BN- doped with metal (eg Ti, Mo, Ta, Cr, Cu, Ni and / or Al) and / or their borides or nitrides, provided that they have good conductivity, are used as starting targets. Targets used. In the process according to the invention, the reactive process is carried out with the addition of Ar and N _{2 in} such a way that the required B: N stoichiometry can be set in the layer.

In general, the process should be carried out in such a way that the carbon content in the cBN layer or in the cBN-containing layer is reduced to values <5at%. In the present invention, a reduced incorporation of carbon in the layer and the required adjustment of the BN ratio is achieved so that the process is carried out in an Ar / N ₂ gas mixture. In analogy to diamond CVD deposition, it would have been expected that an increased oxygen content in the working gas would reduce its proportion in the layer by selective reaction with the carbon. However, it was found that a low oxygen partial pressure is particularly favorable for high proportions of the cubic phase in the layers.

In order to improve the adhesion of the cBN layer or the layer containing cBN to the substrate, a gradient layer should be deposited on the substrate before the actual cBN layer without interrupting the vacuum process, for example by gradually, continuously varying the process gas composition and the process conditions .

In general, it has also proven to be advantageous if the substrate holder is coated with a layer of BC / BN (boron carbide and / or boron nitride), preferably with a thickness of 0.1 to 0.5 μm, before coating.

In addition to other existing possibilities, either the high-frequency sputtering method or sputtering or the direct voltage (DC) magnetron method should be used for the method according to the invention.

If the high-frequency sputtering process is used, it is recommended that the high-frequency coupling be coupled on the target and also on the substrate side, or alternatively, direct voltage (DC) on the substrate side, with a negative bias voltage ( Bias voltage) in the range of 100 to 1,000 V (for example 300 to 500 V), there is an area-related power between 3 and 17 W / cm ² (e.g. 6 W / cm ² ) on the target and an area-related power between 1 and 11 W / cm ² (for example 2 W / cm ² ) is applied, the substrate temperature is kept at a temperature in the range between 30 ° C and 500 ° C (for example at an equilibrium temperature of 350 ° C), an Ar / N ₂ gas mixture, the N ₂ content in the gas mixture being 5% to close to 100% (eg 10 to 70%), and the process pressure being set in the range from 1 to 50 µbar (eg 20 µbar).

If the DC voltage magnetron method with UBM (unbalanced magnetron) is used, it is recommended that an area-related power of 2 to 13 W / cm ² (e.g. 5 W / cm ² ) be applied to the target and an area-related power of 0 to the substrate , 4 to 8 W / cm ² (eg 1 W / cm ² ), the substrate holder, to which a bias voltage is to be applied, rotatably supported, the process pressure set in the range from 1 to 10 µbar (eg 4 µbar) and to use an Ar / N ₂ gas mixture with N ₂ contents between 10 and close to 100% (eg 50% gas flow) as process gas. In this variant of the method according to the invention, in which the DC magnetron method is used, it is possible to apply a high-frequency bias (HF bias) or a DC bias to the substrate holder to create. If a DC voltage bias is applied to the substrate holder, a bias voltage of 100 - 800 V is advantageous, and if a high frequency bias voltage is applied to the substrate holder, a bias voltage would be in the range of 100 V to 1,000 V (e.g. from 200 V) up to 500 V) should preferably be applied.

Expediently, the process should also be run in a generally magnetic field-assisted manner in order to favor the production of the desired layers. This magnetic field support is expediently carried out in such a way that flux densities of approximately 4-7 mT are achieved by means of a coil installed in the recipient, or the additional magnetic field support is achieved by installing additional magnets or electromagnetic coils in such a way that maximum ion current compression takes place with respect to the substrate.

The process according to the invention is very economical, inexpensive and does not have all the disadvantages of the prior art, which makes its use extremely advantageous.

An additional advantage of the high-frequency sputtering method and DC magnetron method is that the process heat available in this method is sufficient so that external heating of the samples can be dispensed with.

It should also be mentioned at this point that it is possible and advantageous for certain applications to additionally add boron-containing gases (eg diborane or trimethylborazine) to the process gas mixture and to use different temperatures from 100 ° C. to 600 ° C. in the course of the layer production.

The method according to the invention can advantageously be used in various fields of application. It is advantageous, for example, for the production of wear-resistant tools (e.g. for the coating of mechanically abrasive and / or adhesive-stressed components, such as indexable inserts, milling cutters, drills, pressing and forming tools, as well as bearings and / or bearing components), for the production of with cubic boron nitride (cBN) coated pickups and components for tape guidance of magnetic tapes, for the deposition of cBN for electronic applications, in particular for the production of electronic components with and without doping, for the production of corrosion protection layers and insulation layers and for the production of optical components with a coating and a protective layer against mechanical stress applied.

With the solution according to the invention, it is thus possible for the first time to produce cBN layers from electrically sufficiently conductive boron-containing targets, thus creating the possibility of the extremely inexpensive and advantageous PVD processes mentioned at the outset, such as the DC magnetron process, the high-frequency sputtering process and use others to produce cBN layers or layers containing cBN.

In general, layers consisting of components B, C and N are already known. Their advantage is, inter alia, that they have high mechanical strengths combined with good optical transparency and thus show advantages over the comparatively hard amorphous hydrocarbon layers, but which are not transparent in the visible spectral range. Another advantage of the cBN layer is its low coefficient of friction µ, which is <0.2 and comparable to diamond layers. Thus, such cBN layers have very good sliding properties, so that they can advantageously be used to coat mechanically abrasive and / or adhesively stressed components as well as bearings and / or bearing components. So far, however, it has not yet been possible to generate the hard cubic phase in such layers in a simple, cost-effective manner. CBN and wBN (wurtzit phase) are aimed for in an equivalent manner. The two phases cBN and wBN, whose density is identical, are to be regarded as equivalent insofar as in both Modifications B and N atoms are coordinated fourfold and the same short-range order is present. There are single bonds (sp ₃ state) between all B and N atoms. In addition, the definition cBN can also be understood to mean a nanocrystalline or amorphous material which has an sp ₃ hybrid bond for the elements B and N. In the present invention, cBN means only the hard phases cBN and wBN.

The method according to the invention is explained in more detail using the following exemplary embodiments.

Examples:

It has been shown that various process controls are suitable for producing wear-resistant layers of cubic boron nitride.

In the examples listed here, a target cleaning phase and a simultaneous ion etching cleaning of the substrates separated by a mechanical shutter (shutter) take place before the actual layer application. In the following exemplary embodiments, the shutters were opened during the layer production.

Silicon wafers (orientations 100 and 111) were used as substrates, as well as steel samples made of 100 Cr6, HSS (High Speed Steel), hard metals, molybdenum and stainless steel.

1. HF sputtering process (high frequency sputtering process)

A method has proven to be particularly advantageous in which the layer is produced in a high-frequency diode sputtering system. The high-frequency coupling takes place on the target and also on the substrate side, with negative bias voltages (bias voltages) in the range of 300-500 V being used. It is also possible to couple direct voltage (DC = direct current) on the substrate side. The desired properties of the layers to be produced are listed below Obtained using a conductive target from B ₄ C. For this purpose, area-related powers of 6 W / cm ^{2 are applied to} the target. It works with powers on the substrate electrode of 2 W / cm ² , the electrodes with a diameter of approx. 170 mm lying opposite each other at a distance of approx. 100 mm. The substrate electrode can be covered with B ₄ C or other B-containing materials. However, a steel plate or other metal can also be used. The process is expediently carried out with magnetic field support in such a way that flux densities of about 4 to 7 mT are produced by means of a coil installed in the recipient, and thus the production of the desired layers is favored. It is not necessary to heat the samples. Due to the heat of the process, the substrates reach an equilibrium temperature of up to 350 ° C during the coating.

The cubic boron nitride layer is created using a gas mixture of Ar and N ₂ , the process pressure being 20 µbar. The N ₂ content in the gas mixture was in the range of 50-70% of the total flow. With this method a 0.8 µm thick cBN layer with the typical properties of the hard cubic boron nitride phase was obtained.

2. DC magnetron method (direct voltage magnetron method)

Another manufacturing process for depositing wear-resistant layers of cubic boron nitride on a substrate is provided according to the invention by the use of a UBM system (UBM = unbalanced magnetron) operated with a DC voltage. In this case, the substrate holder can be operated with an RF bias as well as with a DC bias. A commercial system with a vertically arranged magnetron cathode with additional coils was used for an unbalanced sputtering operation (UBM). The B ₄ C target used had dimensions of 254 mm x 127 mm. The substrate holder, which consisted of a stainless steel plate, was arranged at a distance of 80 mm to 150 mm. The substrate holder was rotatable, so that intermediate layers could also be applied during a vacuum cycle, in which the substrate holder was moved in front of a second magnetron cathode, which was arranged offset by 90 °.

Compared to the previous embodiment with electrodes operated at high frequency, significantly higher deposition rates are achieved here. In addition, relatively large substrates can be coated with such a system. Advantageously, additional magnetic coils and, if necessary, bar magnets are installed in such a way that maximum ion current compression takes place with respect to the substrate.

B ₄ C was used as the target material, an area-related power of 5 W / cm ^{2 being} applied to the target.

• Substrathalter auf Hochfrequenz-Bias: In diesem Fall wurde an das Substrat eine Hochfrequenz-Leistung von 300 W angelegt. Hieraus resultiert eine Bias-Spannung von 200 V bis 500 V. Der Substrathalter, in welchen Hochfrequenz eingespeist wird, steht im Abstand von 80 bis 150 mm zum Target. Auch bei diesem Verfahren erfolgte eine zusätzliche Magneffeldunterstützung, indem zusätzliche Magneten derart eingebaut werden, daß eine maximale lonenstromverdichtung hinsichtlich des Substrates erfolgt. Der Prozeßdruck liegt im Bereich von 4·10^-3 mbar, wobei auch hier mit einer Ar/N₂ Mischung gearbeitet wird. Die N₂-Gehalte liegen bei 80 % (Flow).
• Substrathalter auf Gleichspannungs-Bias: Eine Variante des cBN Prozesses besteht darin, die DC-Magnetronanlage auch auf der Substratseite mit Gleichspannung zu betreiben. In diesem Fall besteht der Substrathalter aus Edelstahl oder anderen geeigneten Metallen. Die anderen Einstellungen bleiben gegenüber dem oben beschriebenen HF-Bias Beispiel gleich. Lediglich an den Substrathalter wird eine DC-Biasspannung von 500 V angelegt. Es zeigte sich, daß Beschichtungsdicken von bis zu 6 µm aus hochisolierenden cBN Schichten auf diese Weise herstellbar sind.

The two possible variants (substrate holder on high-frequency or DC bias) were carried out with the following parameters:

• Substrate holder on high-frequency bias: In this case, a high-frequency power of 300 W was applied to the substrate. This results in a bias voltage of 200 V to 500 V. The substrate holder into which high frequency is fed is at a distance of 80 to 150 mm to the target. This method also provided additional magnetic field support in that additional magnets are installed in such a way that maximum ion current compression takes place with respect to the substrate. The process pressure is in the range of 4 · 10 ^-3 mbar, an Ar / N ₂ mixture also being used here. The N ₂ contents are 80% (flow).
• Substrate holder with DC voltage bias: One variant of the cBN process is to operate the DC magnetron system with DC voltage also on the substrate side. In this case, the substrate holder is made of stainless steel or other suitable metals. The other settings remain the same compared to the RF bias example described above. A DC bias voltage of 500 V is only applied to the substrate holder. It was found that coating thicknesses of up to 6 μm can be produced from highly insulating cBN layers in this way.

As a variant of this exemplary embodiment, a 100 nm to 1,000 nm thick intermediate layer composed of a conductive Ti B (N) and / or B ₄ C layer is applied to the substrate holder and the substrates mounted thereon before the coating.

A comparable conductive layer can also be applied in between for the production of even thicker cBN layers in a very limited thickness.

External heating of the samples can also be dispensed with in the case of the DC magnetron example.

Claims (22)

1. Method of manufacturing wear-resistant layers of cubic boron nitride or wear-resistant layers containing the same, by powdering with high frequency or with direct current in the operational mode as an unbalanced magnetron, in which the plasma is generated by means of DC arc discharges or magnetron cathodes operated by DC, characterised in that there is used as an initial target a target of electrically conductive boron carbide, and the stoichiometry of the layer is set with reactive process guidance with the supply of N₂ and Ar.
2. Method according to claim 1, characterised in that a target is used made of boron carbide in the composition range of 70 to 90atomic% boron and 10 to 30atomic% carbon.
3. Method according to claim 2, characterised in that a target made of B₄C is used.
4. Method according to one or more of claims 1 to 3, characterised in that the carbon content of the deposited layer is reduced to values beneath 5atomic%.
5. Method according to one or more of claims 1 to 4, characterised in that, in order to improve the adhesion of the layer of cubic boron nitride, a base is deposited by staged or continuous alteration of the process gas composition and of the process conditions on the substrate, without interrupting the vacuum.
6. Method according to one or more of claims 1 to 5, characterised in that the substrate carrier is covered, before coating, with a layer of boron carbide/boron nitride in the layer thickness range of 0.1 to 0.5 µm.
7. Method according to one or more of claims 1 to 6, characterised in that, in high frequency powdering, the high frequency coupling is effected on the target and on the substrate side, a negative bias in the range of 100 to 1,000 volts being used, a surface power between 3 and 17 W/cm² being applied to the target and between 1 and 11 W/cm² to the substrate, the substrate temperature being kept during coating in a range between 30°C and 500°C, a process gas of an Ar/N₂ gas mixture with 5% to near 100% N₂ is used, and the process pressure is set between 1 and 50 µbar.
8. Method according to claim 7, characterised in that the substrate is supplied with direct current.
9. Method according to claim 7 and 8, characterised in that a surface power of 6 W/cm² is applied to the target, and of 2 W/cm² to the substrate, the substrate is kept during coating at an equilibrium temperature of 350°C and a process gas mixture of Ar and N₂ with 10 to 70% N₂ and a process pressure of 20 µbar is used.
10. Method according to one or more of claims 1 to 6, characterised in that, in DC magnetron powdering in the mode of operation as an unbalanced magnetron, a surface power of 2 to 13 W/cm² is applied to the target and of 0.4 to 8 W/cm² to the substrate, the substrate carrier is mounted to rotate, the process pressure lies in the range of 1 to 10 µbar, there is used as a process gas an Ar/N₂ gas mixture with 10 to near 100% N₂ in the range between 1 and 10 µbar, and there is applied to the substrate carrier either a bias voltage of 100 - 800 volts DC or from 100 to 1,000 volts of high frequency voltage.
11. Method according to claim 10, characterised in that a surface power of 5 W/cm² is applied to the target and of 1 W/cm² to the substrate, a process gas flow with 50% N₂ and a process pressure of 4 µbar are set.
12. Method according to one or more of claims 1 to 11, characterised in that the process is operated with a reinforced magnetic field, a coil being incorporated in the recipient with flux densities of 4 to 7 mT.
13. Method according to claim 12, characterised in that an additional magnetic field reinforcement is achieved if additional magnets or electromagnetic coils are incorporated so that a maximum ion flow density is achieved on the substrate.
14. Method according to one or more of claims 1 to 13, characterised in that boron-containing gases are added to the process gas.
15. Method according to claim 14, characterised in that diborane or trimethylborazine is added to the process gas.
16. Method according to one or more of claims 1 to 15, characterised in that during coating temperatures of 100°C to 600°C are used.
17. Application of the method according to one or more of claims 1 to 16 to the manufacture of wear-resistant tools.
18. Application according to claim 17 to components subject to mechanical abrasive and/or adhesive stress, and to bearings and/or bearing components.
19. Application according to one or more of claims 1 to 16 for manufacturing sound pick-ups coated with cubic boron nitride and components for guiding the tape of tape recorders.
20. Application according to one or more of claims 1 to 16 for depositing cubic boron nitride for electronic applications, particularly for manufacturing electronic components with and without doping.
21. Application according to one or more of claims 1 to 16 for manufacturing anti-corrosion layers and insulating layers.
22. Application according to one or more of claims 1 to 16 for manufacturing optical components with a blooming layer and a protective layer against mechanical stress.